The invention relates to a method of closing a pressure vessel and more particularly to a method of closing a pressure vessel containing radioactive materials.
Commercial nuclear power plants are presently operated for up to twelve to eighteen months or more to generate electrical power and then are taken off-line on scheduled inspection, maintenance and/or refueling outages. In the course of routine refueling outages, about a third of the spent fuel assemblies in the cores of nuclear reactor pressure vessels may need to be replaced with fresh fuel assemblies and the other fuel assemblies may need to be repositioned in the cores. In the course of other outages, the fuel assemblies may remain in the core but must be exposed. The fuel assemblies contain enriched uranium and perhaps other fissile materials. Thus, personnel working around the pressure vessels must be protected from excessive radiation exposure. Each step of an outage, whether or not on a critical path schedule, must be performed as rapidly and as safely as possible in order to maintain a short outage time.
After a pressure vessel has been inspected, serviced or refueled, the pressure vessel must be closed so that the plant can return to on-line power generation operations. Outage schedules generally allocate at least about ten hours or more and require five or more workers to move bulky, heavy stud tensioning equipment into the high radiation environments surrounding the nuclear reactor pressure vessels in reactor buildings, manipulate the equipment around the pressure vessel flanges to tension the closure stud nuts mounted on closure studs holding reactor pressure vessel heads to reactor pressure vessel bodies, and then remove the tensioning equipment from the reactor buildings. The tensioning step in commercial plants (which typically involves the simultaneous use of several multi-stud tensioners) requires two hours or more and may expend up to 500 millirem or more of radiation. See, in this regard, U.S. Pat. Nos. 3,851,906; 4,223,575 and 6,167,764, which generally disclose the types of tensioners and turn out tools and methods used in the nuclear power industry to close reactor pressure vessels in order to access the fuel assemblies in their core regions.
It is an object of the present invention to provide a method for closing pressure vessels in radioactive environments more rapidly than is possible using the equipment and current best practices of the nuclear power industry. It is a further object to provide a method for closing the pressure vessels with reduced personnel radiation exposure.
With these objects in view, the present invention resides in a method of closing a pressure vessel, comprising the steps of: placing a closure head having a flange over and spaced from a flange of a pressure vessel body containing radioactive fuel assemblies; installing threaded closure studs in closure stud holes of the pressure vessel body flange; attaching hydraulic nut assemblies to the closure studs; connecting hydraulic hoses between the hydraulic nut assemblies and a pressure source; simultaneously pressurizing the hydraulic nut assemblies from the pressure source to simultaneously apply a force to the closure studs sufficient to seat the spaced apart closure head flange on the pressure vessel body flange and to elongate the closure studs; locking the hydraulic nut assemblies in place over the closure head flange while the force is applied to the closure studs and the closure studs are elongated; and then releasing the hydraulic pressure on the hydraulic nut assemblies while the hydraulic nut assemblies are locked in place over the closure head flange and the closure studs are elongated.
In a preferred practice, where the pressure vessel contains a removable upper core support assembly supported by springs on the fuel assemblies, the step of placing the closure head over and spaced from the pressure vessel body flange includes supporting the closure head on the upper core support assembly spring supported by the fuel assemblies.
In another preferred practice, the closure studs are installed while the closure head flange is over and spaced from the pressure vessel body flange. In this practice, the hydraulic nut assemblies or structural members of the assemblies may be attached to the closure studs before the closure studs are installed.